The present invention relates to a percussion drill bit for drilling bores in rock. A typical percussion drill bit comprises a steel drill body, means at one end of the drill body for connecting the bit to a percussive unit such as a down-the-hole hammer or a drill string for a jack hammer, and a plurality of inserts embedded in the other end of the drill body.
At least the peripherally located inserts, or gauge row inserts, comprise a cemented carbide body having a rear mounting portion embedded in the drill body and a front end protruding from the drill body. A polycrystalline diamond layer is provided on the converging end of the carbide body, the layer having an end edge disposed in the vicinity of the maximum diameter of the insert.
Known drill bits that are provided with diamond-coated gauge inserts often have a disadvantage in that premature failures occur due to fatigue of the diamond-coating. A reason for the failure is that the point of contact between the wall of the bore and the gauge insert is close to, or on, the rear edge area of the diamond coating. The coating is very thin at that area and thus has a comparatively poor resistance to wear. The known drill bits more or less drill the bore diameter with the thinnest part of the gauge insert since their gauge inserts are inclined by about 35.degree. relative to the center line of the drill bit.